Color image forming apparatus

ABSTRACT

Registration patterns are formed plural times at arbitrary positions in the period determined by the conveying drawing fluctuation period detector when forming registration patterns. The average of the color deviation amount obtained from these plural registration patterns is the color deviation amount. As a result, the color deviation amount can be detected accurately. By detecting the accurate color deviation amount, the precision of color deviation correction is enhanced. It hence presents a color image forming apparatus capable of obtaining images of high print quality.

FIELD OF THE INVENTION

The present invention relates to a color image forming apparatus havinga color deviation detecting function in a registration mechanism ofcolor image forming apparatus of tandem engine type.

BACKGROUND OF THE INVENTION

In a conventional image forming apparatus of electrophotographic type, auniform electric charge of about 1 millicoulomb per square centimeter isapplied on the surface of a photosensitive drum. This surface of aphotosensitive drum is exposed depending on the image information, andthe electric charge of its irradiated portion only is released to thephotosensitive drum substrate, and an image (electrostatic latent image)is formed as a result of distribution of electric charge. Thiselectrostatic latent image is developed by coloring charged particles(toner particles), and a powder image is formed (toner development). Thepowder image is transferred onto a sheet material or the like. Thetransferred powder image is fused and fixed by heat or other energy, andan image is formed.

Recently, on the other hand, the image by the image forming apparatus ofelectrophotographic type is becoming colorful. As the color imageforming apparatus, a color image forming apparatus of tandem engine typeis proposed. This color image forming apparatus includes a plurality ofimage forming stations having a photosensitive drum. The plurality ofimage forming stations are image forming stations for forming cyanimage, magenta image, yellow image, and, preferably, black image on eachphotosensitive drum. The powder image formed on each photosensitive drumis overlaid on an intermediate transfer material at transfer position ofeach color powder image, and transferred and synthesized. In the colorimage forming apparatus of tandem engine type, the image can be formedat high speed because each image is formed parallel in each color.

In the case of color image forming apparatus, however, each powder imageformed in a different image forming station may be deviated in positiondue to positioning error between image forming stations, resulting incolor deviation. To develop a color image forming apparatus of highquality, such color deviation is a serious problem, and the techniquefor correcting color deviation (registration) is required.

FIG. 8 is a block diagram of a conventional color image formingapparatus, and FIGS. 9A, B, C, D, E are diagrams showing types of colordeviation in a general color image forming apparatus.

As shown in FIG. 8, a conventional color image forming apparatuscomprises four image forming stations Pa, Pb, Pc, Pd arranged parallelin contact with the top of an intermediate transfer belt 7 which isrotated by drive rollers 14 a, 14 b. Each image forming station hasphotosensitive drums 1 a, 1 b, 1 c, 1 d contacting with the intermediatetransfer belt 7 mounted on the color image forming apparatus main bodyrespectively. Around the photosensitive drums 1 a, 1 b, 1 c, 1 d,chargers 2 a, 2 b, 2 c, 2 d are disposed for providing eachphotosensitive drum with an electrostatic charge. On the top of theimage forming stations Pa, Pb, Pc, Pd, there is an exposure unit 3 whichis a scanning optical system for forming an electrostatic latent imageby exposing the light depending on the image information to thephotosensitive drums 1 a, 1 b, 1 c, 1 d charged by the charger unit. Theexposure unit 3 includes an exposure unit 3K for the photosensitive drum1 a, an exposure unit 3C for the photosensitive drum 1 b, an exposureunit 3M for the photosensitive drum 1 b, and an exposure unit 3Y for thephotosensitive drum 1 d. A color deviation detector 13 is disposed nearthe drive roller 14 a.

Each one of the photosensitive drums 1 a, 1 b, 1 c, 1 d is surroundedby:

developing units 4 a, 4 b, 4 c, 4 d for developing the electrostaticlatent image formed by exposure by coloring toner particles, and forminga powder image,

transfer units 5 a, 5 b, 5 c, 5 d for transferring and synthesizing byoverlaying each powder image formed by the developing units 4 a, 4 b, 4c, 4 d on the intermediate transfer belt 7 at the transfer position ofeach color powder image, and

cleaning units 6 a, 6 b, 6 c, 6 d for cleaning and removing the residualtoner particles on the surface of the photosensitive drums 1 a, 1 b, 1c, 1 d after the powder images are transferred on the ntermediatetransfer belt 7 by the transfer units 5 a, 5 b, 5 c, 5 d.

The intermediate transfer belt 7 moves in the direction of arrow A shownin FIG. 8, and the photosensitive drums 1 a, 1 b, 1 c, 1 d rotate in thedirection of arrow B shown in FIG. 8 without sliding on the intermediatetransfer belt 7.

In the conventional color image forming apparatus having suchconstitution, the image forming operation is described below.

First, at the image forming station Pa, the surface of thephotosensitive drum 1 a is uniformly charged with an electrostaticcharge by the charger 2 a.

An electrostatic latent image corresponding to the image information ofblack component is formed on the photosensitive drum 1 a by the exposureunit 3K.

This electrostatic latent image is developed on the photosensitive drumla as a powder image by black toner particles by the developing unit 4a.

This powder image is transferred on the intermediate transfer belt 7 asa black toner image by the transfer unit 5 a.

The surface of the photosensitive drum 1 a after transfer process iscleaned by the cleaning unit 6 a, and residual toner particles areremoved to be ready for next image formation.

Parallel to the timing of forming the black toner image, at the imageforming station Pb, the surface of the photosensitive drum 1 b isuniformly charged with an electrostatic charge by the charger 2 b, and

An electrostatic latent image corresponding to the image information ofcyan component is formed on the photosensitive drum 1 b by the exposureunit 3C.

This electrostatic latent image is developed on the photosensitive drum1 b as a powder image by cyan toner particles by the developing unit 4b, and

It is laid over the black toner image formed on the intermediatetransfer belt 7, and a synthetic toner image is formed.

Similarly, thereafter, a magenta toner image is overlaid by the imageforming station Pc, and a yellow toner image by the image formingstation Pd sequentially on the intermediate transfer belt 7. In thisway, the synthetic toner image is formed by overlaying four color tonerimages on the intermediate transfer belt 7.

After completion of formation of the synthetic toner image, a sheetmaterial 9 of paper or the like is supplied in between the intermediatetransfer belt 7 and a transfer roller 11 from a paper feed cassette 10through a paper feed roller 8. The transfer roller 11 is disposed at aposition contacting with the intermediate transfer belt 7 for insertingthe sheet material 9 between it and the intermediate transfer belt 7.When the sheet material 9 is supplied, the synthetic toner image istransferred on the sheet material 9. Then, after being heated and fixedby a fixing unit 12, a color image is formed on the sheet material 9.

In such image forming apparatus of tandem engine type, however, colordeviation may occur in the following cases:

1) Unstable temperature when turning on the power.

2) Exchange of image forming stations Pa, Pb, Pc, Pd.

3) Setting condition of the image forming apparatus.

4) Deviation of mounting of the image forming stations Pa, Pb, Pc, Pdand scanning optical system due to temperature changes in the apparatus.

FIGS. 9A to E depict modes of color deviation.

FIG. 9A shows deviation of sub-scanning position by moving parallel tothe moving direction A of the intermediate transfer belt 7. In thediagram, Ra indicates a correct position, and Ea shows a deviatedposition, schematically.

FIG. 9B shows deviation of main scanning position by moving parallel tothe scanning direction of the exposure unit 3 (the direction vertical tothe moving direction A of the intermediate transfer belt 7). In thediagram, Rb indicates a correct position, and Eb shows a deviatedposition, schematically.

FIG. 9C shows a skew error of the image inclined obliquely to thescanning direction of the exposure unit 3. In the diagram, Rc indicatesa correct position, and Ec shows a deviated position, schematically.

FIG. 9D shows a magnification error of deviation of magnification factorof the exposure unit 3 in the scanning direction among the image formingstations Pa, Pb, Pc, Pd. In the diagram, Rd indicates a correctposition, and Ed shows a deviated position, schematically.

FIG. 9E shows a curvature error of the image curving in the scanningdirection of the exposure unit 3. In the diagram, Re indicates a correctposition, and Ee shows a deviated position, schematically.

Main factors of color deviation shown in FIG. 9A and FIG. 9B includedeviation of mounting of image forming stations Pa, Pb, Pc, Pd andexposure unit 3, and deviation of mounting of lens and mirror (notshown) for composing the scanning optical system of the exposure unit 3.Main factors of color deviation shown in FIG. 9C include deviation ofmounting angle of rotary shafts of photosensitive drums 1 a, 1 b, 1 c, 1d of the image forming stations Pa, Pb, Pc, Pd, and deviation ofmounting angle of the exposure unit 3. Main factors of color deviationshown in FIG. 9D include the error of optical path length from thescanning optical system of the exposure units 3K, 3C, 3M, 3Y to thesurface of the photosensitive drums 1 a, 1 b, 1 c, 1 d. Main factors ofcolor deviation shown in FIG. 9E include deviation of mounting of lensand mirror (not shown) for composing the scanning optical system ofexposure units 3K, 3C, 3M, 3Y.

Accordingly, when turning on the power source, when exchanging the imageforming stations, or when the temperature varies in the apparatus, it isrequired to correct color deviation for matching the position of colorimages depending on the extent of deviation.

The correction is carried out in the following procedure:

A reference pattern (called registration pattern hereinafter) is markedpreliminarily on the intermediate transfer belt 7;

The registration pattern is detected by a color deviation detector 13composed of a plurality of sensors;

Extents of five types of color deviation are calculated from the resultsof detection; and

The color deviation is corrected by matching the position of each colorimage depending on the extent of deviation.

The operation of color deviation detection and color deviationcorrection of the conventional color image forming apparatus isdescribed below while referring to FIGS.10, 11, 12, and 13.

FIG. 10 is a block diagram of a conventional color deviation detector.FIG. 11 is a configuration of the registration pattern and positiondeviation detector on the conventional intermediate transfer belt. FIG.12 is a diagram showing a configuration of the registration pattern andposition deviation detector on the conventional intermediate transferbelt, and an output signal of the color deviation detector.

In FIG. 10, the color deviation detector 13 disposed on the intermediatetransfer belt 7 for detecting deviation of position of the registrationpattern is composed of:

a light source 31 such as lamp and laser,

a sensor 33, being a charge coupled device (CCD) used as an image sensorfor detecting registration pattern, and

a lens 32 for focusing the image of the registration pattern illuminatedby the light source 31 on the sensor 33.

The color deviation detector 13 consists of two color deviationdetectors (13 a and 13 b) disposed near the scanning start position andnear the scanning end position of the exposure unit 3. The sensors ofthe two color deviation detectors 13 a, 13 b are arranged on a lineorthogonal to the moving direction A of the intermediate transfer belt 7as shown in FIG. 11.

The patterns 34, 35, 36, 37 as shown in FIG. 11 are toner images ofpredetermined lines and figures transferred on the intermediate transferbelt 7 in each color at predetermined intervals by the image formingoperation.

In such constitution, the color deviation detecting operation is tomeasure the extent of position deviation (color deviation) of the tonerimages 34, 35, 36, 37 of each color transferred on the intermediatetransfer belt 7 as shown in FIG. 11 by the color deviation detector 13 aand color deviation detector 13 b.

Concerning deviation of sub-scanning position shown in FIG. 9A, as shownin FIG. 12A, position deviation ΔY1 (ΔY1=ΔT1·v) of each color iscalculated from the time difference ΔT(ΔT1=T−T1) between the passingtime T1 of the registration pattern of each color on the intermediatetransfer belt 7 through the CCD 33 a in the color deviation detector 13a and the predetermined design value T, and the moving speed v of theintermediate transfer belt 7.

Concerning deviation of main scanning position shown in FIG. 9B, asshown in FIG. 13A, position deviation of each color is calculated fromthe pixel position difference ΔX1 when the scanning start position ofthe registration pattern of each color on the intermediate transfer belt7 passes through the CCD 33 a in the color deviation detector 13 a.

Concerning skew error shown in FIG. 9C, as shown in FIG. 12B, skew errorΔY2(ΔY2=ΔT2·v) of each color is calculated from the time difference(ΔT2) of the registration pattern of the same color on the intermediatetransfer belt 7 passing through the CCD 33 a and CCD 33 b in the colordeviation detector 13 a and color deviation detector 13 b, and themoving speed v of the intermediate transfer belt 7.

Concerning magnification error shown in FIG. 9D, as shown in FIG. 13Aand FIG. 13B, magnification error (ΔX3=ΔX2−ΔX1) of each color iscalculated from the pixel position difference (ΔX2, ΔX1) of the scanningstart position and scanning end position of the registration pattern ofthe same color on the intermediate transfer belt 7 passing through theCCD 33 a and CCD 33 b in the color deviation detector 13 a and colordeviation detector 13 b.

Concerning curvature error shown in FIG. 9E, it is not possible tomeasure accurately by the conventional color deviation detectingoperation. Therefore, there is no other method of decreasing thecurvature error than the method of improving the assembling precision ofthe lens and others in the exposure unit 3.

Depending on the four types of color deviation extent detected in theprocedure herein, the color deviation correcting operation is carriedout as follows.

FIG. 14 is a block diagram of a color deviation correcting mechanism ofthe scanning optical system in the exposure unit in the prior art.

In FIG. 14, the scanning optical system of the exposure unit 3 disposedon the photosensitive drum 1 comprises:

a polygon motor 61,

a polygon mirror 62 mounted on the shaft of the polygon motor 61,

a pair of folded mirrors 63, 64 capable of moving freely in thelongitudinal and lateral direction by an adjusting actuator 66 and anadjusting actuator 67, with the mirror surfaces held at a right angle,and

a folded mirror 65 for finally emitting the scanning beam to thephotosensitive drum 1.

The adjusting actuator 66 moves the folded mirrors 63, 64 horizontallyin the longitudinal direction (direction of arrow C in FIG. 14). Theadjusting actuator 67 moves the folded mirrors 63, 64 in the verticaldirection (direction of arrow D in FIG. 14). As actuators for theseadjustments, linear step actuators having step motors as the drivesource for moving linearly in gradual steps are used. The image data isoptically modulated by a semiconductor laser (not shown), and fed intothe polygon mirror 62. The scanning beam reflected by the polygon mirror62 is sequentially reflected by the folded mirrors 63, 64, 65, and isemitted to the surface of the photosensitive drum 1. The irradiationposition of the scanning beam is controlled by the polygon motor 61,adjusting actuator 66, and adjusting actuator 67 which are drivenindependently.

The sub-scanning position deviation shown in FIG. 9A and main scanningposition deviation shown in FIG. 9B are corrected by the timingcorrection of scanning start of the exposure unit 3 of each color, phasecontrol of timing signal of scanning start, and phase control of polygonmotor. The skew error shown in FIG. 9C and magnification error shown inFIG. 9D are corrected by adjustment of irradiation position andadjustment of optical path length by the adjusting actuator 66 andadjusting actuator 67.

In the conventional constitution, however, due to eccentricity of thedrive unit of the intermediate transfer belt, shaft run-out of the driveunit, or meandering of intermediate transfer belt, the toner imagetransfer position on the intermediate transfer belt is not moving atuniform speed (zero acceleration). Accordingly, the position of thetoner image transferred on the intermediate transfer belt is notconstant. Therefore, the registration pattern for inspecting the colordeviation amount is transferred in a deviated state, and thereby theregistration pattern oscillates.

When such registration pattern is used in detection of extent of colordeviation, an extent of color deviation different from the actual statusis detected. As a result, in spite of correction, to the contrary, thecolor deviation amount may be increased to lower the print quality.

Generally, color deviation consists of:

1) DC component occurring any time due to mounting error of intermediatetransfer belt or the like, and

2) AC component occurring periodically due to uneven rotation ofphotosensitive drum or the like; and usually these two factors are mixedto cause color deviation.

The color deviation AC component occurs periodically (withoutscattering) like the sine curve, and the DC component is a colordeviation occurring any time.

If the DC component is detected at the position largest in the amount ofthe AC component, the sum of AC component deviation+DC componentdeviation is detected, and accurate deviation of DC component cannot bemeasured.

Herein, seeing that the AC component occurs alternately in the plusdirection and minus direction, with a periodic rebound, the AC componentcan be canceled by determining the period of AC component, measuring DCcomponent plural times in the period, and averaging.

As a result, effects of AC components are eliminated, and an accurate DCcomponent can be calculated.

That is, it is an object of the invention to present a color imageforming apparatus capable of obtaining images of high print quality, byeliminating only AC components from the mixed color deviation of ACcomponents and DC components, detecting the DC components, drawing theregistration pattern at high precision, and correcting the colordeviation accurately by the registration pattern drawn at highprecision.

The invention is devised to solve the above problems, and hence providesa color image forming apparatus capable of correcting the colordeviation accurately and obtaining images of high print quality, bydrawing the registration pattern at high precision.

SUMMARY OF THE INVENTION

To solve the problems, the color image forming apparatus of theinvention comprises:

a) an image conveying unit for conveying an image by holding it on thesurface,

b) a plurality of image forming units disposed for each color forforming the image corresponding to each color image information on theimage conveying unit,

c) a registration pattern generator for controlling formation ofregistration pattern of each color on the image conveying unit by theimage forming units at the time of detection operation of colordeviation,

d) a color deviation detector for detecting color deviation of the imagefrom the registration pattern, and

e) a conveying drawing fluctuation period detector for detecting periodof drawing fluctuation in the image conveying unit.

In this constitution, when forming the registration pattern, theregistration pattern is formed plural times at an arbitrary position inthe period determined by the conveying drawing fluctuation perioddetector. The average of the color deviation values obtained from theplural registration patterns is obtained as the color deviation amount,so that a more accurate color deviation can be detected. By detectingthe more accurate color deviation, the precision of correction of colordeviation is enhanced. As a result, the color image forming apparatuscapable of obtaining images of high print quality is presented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a color image forming apparatus in anembodiment of the invention.

FIG. 2 is an outline drawing of the color image forming apparatus in theembodiment of the invention.

FIG. 3 is a function block diagram of the color image forming apparatusin the embodiment of the invention.

FIG. 4A is an example of ladder pattern for explaining the embodiment ofthe invention.

FIG. 4B is a graph showing a case without conveying fluctuation in thetime series.

FIG. 5A is a diagram showing a ladder pattern formed on an intermediatetransfer belt.

FIG. 5B is a graph showing conveying fluctuation and its period of theladder pattern in FIG. 5A in the time series.

FIG. 5C is a graph showing an example of determining a plurality ofsampling positions S1 to S5 in the period T of the ladder pattern inFIG. 5B.

FIG. 6 is a graph showing conveying fluctuation and its period inembodiment 2 of the invention in the time series.

FIG. 7 is a graph showing conveying fluctuation and its period inembodiment 3 of the invention in the time series.

FIG. 8 is a block diagram of a conventional color image formingapparatus.

FIGS. 9A, B, C, D, and E are diagrams showing various types of colordeviation in a general color image forming apparatus.

FIG. 10 is a block diagram of a conventional color deviation detector.

FIG. 11 is a configuration of registration pattern and positiondeviation detector on a conventional intermediate transfer belt.

FIG. 12A is a configuration of registration pattern and positiondeviation detector on a conventional intermediate transfer belt.

FIG. 12B is a diagram showing an output signal of a color deviationdetector in FIG. 12A.

FIG. 12C is a diagram showing output signals of a sensor 13 a and asensor 13 b of the color deviation detector in FIG. 12A.

FIG. 13A is a configuration of registration pattern and positiondeviation detector on a conventional intermediate transfer belt.

FIG. 13B is a diagram showing an output signal of a color deviationdetector in FIG. 13A.

FIG. 14 is a block diagram of color deviation correcting mechanism ofthe scanning optical system in a conventional exposure unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The color image forming apparatus according to embodiments 1 to 9 of theinvention is described below while referring to the accompanyingdrawings.

(Embodiment 1)

FIG. 1 shows a constitution of a color image forming apparatus in thisembodiment, and FIG. 2 is an outline drawing of the color image formingapparatus of the embodiment in FIG. 1. FIG. 3 is a function blockdiagram of the color image forming apparatus in the embodiment of theinvention. In the color image forming apparatus of the embodiment, sameparts as in the prior art in FIG. 8 are identified with same referencenumerals, and their description is omitted.

It is explained below by referring to FIG. 1 and FIG. 3. This embodimentis similar to the conventional constitution shown in FIG. 8, except thata conveying drawing fluctuation period detector 71 and a registrationpattern generator 72 are added.

The conveying drawing fluctuation period detector 71 is composed of:

a ladder pattern forming unit 71A for forming a ladder pattern ofuniform interval and uniform width, and

a period detector 71B for determining the transition of period andextent of drawing fluctuation by measuring the ladder pattern formed onthe intermediate transfer belt 7.

The ladder pattern formed by the ladder pattern forming unit 71A isformed on the intermediate transfer belt 7 in the same procedure as inthe conventional registration pattern formation. The period detector 71Bdetects the conveying fluctuation by the same procedure as in theconventional color deviation detector 13, and the transition of periodand extent of drawing fluctuation is determined by the frequencyanalysis technique such as Fourier analysis.

The transition of period and extent of drawing fluctuation is detectedin each color.

The registration pattern generator 72 controls the exposure unit 3 forforming electrostatic latent images on the photosensitive drums 1 a, 1b, 1 c, 1 d for forming the registration pattern. The color deviationdetector 13 detects the extent of position deviation (color deviation)of each color toner image formed on the intermediate transfer belt 7 bythe individual color image forming stations Pa, Pb, Pc, Pd. A colordeviation corrector 73 corrects color deviation corresponding to thecolor deviation detected by the color deviation detector 13 as shown inFIGS. 9A, B, C, and D.

The color deviation detector 13 is same as the conventional constitutionshown in FIG. 10, and description of its constitution and operation isomitted.

The color deviation corrector 73 is same as the conventionalconstitution shown in FIG. 14, and description of its constitution andoperation is omitted.

FIG. 4A shows an example of ladder pattern formed by the ladder patternforming unit. In FIG. 4B, the axis of ordinates X denotes the amount ofdrawing interval fluctuation due to speed fluctuation or the like at anarbitrary point on the intermediate transfer belt, and the axis ofabscissas represents the time. Without drawing fluctuation, for example,by drawing and measuring a ladder pattern with intervals Wt, a graph asshown in FIG. 4B would be obtained. Actually, however, a drawingfluctuation occurs at a specific interval combining the period of theintermediate transfer belt and each constituent element (gear, pulley,etc.) in the drive unit of the photosensitive materials, and the ladderpattern on the intermediate transfer belt is as shown in FIG. 5A. InFIG. 5B, T denotes the period of drawing fluctuation, and A and B aremaximum values of fluctuation in the plus direction and minus direction,respectively. Such drawing fluctuations have effects directly also ondrawing of the registration pattern, and the registration pattern isdrawn in a state deviated by A in the plus direction and B in the minusdirection in a worst case. As a result, the color deviation amountincludes errors of A to −B from the true deviation. This drawingfluctuation, by nature, does not scatter in the plus direction or minusdirection, but appears repeatedly in the plus direction and minusdirection, or in one direction only. Accordingly, as shown in FIG. 5C, aplurality of sampling positions S1 to S5 are determined in the period Tdetermined in the conveying drawing fluctuation period detector. At thedetermined positions, the registration pattern is drawn, and the colordeviation detector 13 detects the drawn pattern. By averaging thedetected values of the plurality of patterns, the error is controlled atleast smaller than A to −B. It is therefore possible to detect the colordeviation more precisely. The sampling positions and the number thereofmay be set properly depending on the actual state of drawingfluctuation.

(Embodiment 2)

FIG. 6 is a graph showing the conveying fluctuation and its period inthe time series in embodiment 2 of the invention.

The apparatus constitution of the image forming unit of the color imageforming apparatus in this embodiment is same as in embodiment 1 shown inFIG. 1, and its description is omitted.

The function block diagram of the color image forming apparatus of thisembodiment is same as in embodiment 1 shown in FIG. 3, and itsdescription is omitted.

In FIG. 6, the axis of ordinates X denotes the amount of drawingfluctuation due to speed fluctuation or the like at an arbitrary pointon the intermediate transfer belt, and the axis of abscissas representsthe time.

As shown in FIG. 6, a drawing fluctuation symmetrical in a half periodas a typical sine curve may appear. In this case, in particular, theperiod is equally divided and sampling positions are determined inconsideration of its nature. As shown in FIG. 6, the average of thevalue of sampling position S1 and value of sampling position S3 is 0,and also the average of the value of sampling position S2 and value ofsampling position S4 is 0. Accordingly, at the equally divided points ofthe period T determined in the conveying drawing fluctuation perioddetector, by drawing the registration pattern and averaging the resultsof measurement, the color deviation extent can be accurately detectedwithout having effects of the drawing fluctuation.

(Embodiment 3)

FIG. 7 is a graph showing the conveying fluctuation and its period inthe time series in embodiment 3 of the invention.

The apparatus constitution of the image forming unit of the color imageforming apparatus in this embodiment is same as in embodiment 1 shown inFIG. 1, and its description is omitted.

The function block diagram of the color image forming apparatus of thisembodiment is same as in embodiment 1 shown in FIG. 3, and itsdescription is omitted.

In FIG. 7, the axis of ordinates X denotes the amount of drawingfluctuation due to speed fluctuation or the like at an arbitrary pointon the intermediate transfer belt, and the axis of abscissas representsthe time. The drawing fluctuation, by nature, does not scatter in theplus direction or minus direction, but appears repeatedly in the plusdirection and minus direction, or in one direction only. In other words,there is always a region S of small drawing fluctuation within theperiod T as shown in FIG. 7. Accordingly, in the region S of smalldrawing fluctuation in the period determined in the conveying drawingfluctuation period detector, the registration pattern is drawn. Thus,the registration pattern hardly influenced by the drawing fluctuationcan be drawn, so that an accurate color deviation amount can bedetected.

(Embodiment 4)

The apparatus constitution of the image forming unit of the color imageforming apparatus in this embodiment is same as in embodiment 1 shown inFIG. 1, and its description is omitted.

The function block diagram of the color image forming apparatus of thisembodiment is same as in embodiment 1 shown in FIG. 3, and itsdescription is omitted.

The period for composing the drawing fluctuation generally appears in aplurality, but principal periods excluding the noise can be reduced toseveral ones. Principal periods of drawing fluctuation can be consideredas least common multiples of these plurality of periods (not shown).Accordingly, in the periods of the least common multiples of theplurality of principal periods, the registration pattern is drawn in thesame manner as in embodiment 1, and the detected values are averaged.Thus, the color deviation amount can be detected at high precision.

(Embodiment 5)

The apparatus constitution of the image forming unit of the color imageforming apparatus in this embodiment is same as in embodiment 1 shown inFIG. 1, and its description is omitted.

The function block diagram of the color image forming apparatus of thisembodiment is same as in embodiment 1 shown in FIG. 3, and itsdescription is omitted.

The period for composing the drawing fluctuation generally appears in aplurality, but principal periods excluding the noise can be reduced toseveral ones. Principal periods of drawing fluctuation can be consideredas least common multiples of these plurality of periods (not shown).Accordingly, in the periods of the least common multiples of theplurality of principal periods, the registration pattern is drawn in thesame manner as in embodiment 2, and the detected values are averaged.Thus, the color deviation amount can be detected at high precision.

(Embodiment 6)

The apparatus constitution of the image forming unit of the color imageforming apparatus in this embodiment is same as in embodiment 1 shown inFIG. 1, and its description is omitted.

The function block diagram of the color image forming apparatus of thisembodiment is same as in embodiment 1 shown in FIG. 3, and itsdescription is omitted.

When the drawing fluctuation is a compound wave of a plurality ofperiods, the period and characteristic of the compound wave are mostinfluenced by the wave of the largest amplitude (drawing fluctuation)(not shown). In frequency analysis, each period is handled as a waveformsymmetric in a half period such as sine wave and cosine wave.Accordingly, regarding also the sine wave (cosine wave) having thelargest amplitude (drawing fluctuation) among the drawing fluctuations,in this sine wave cosine wave), the registration pattern is drawn in thesame manner as in embodiment 1, and the detected values are averaged.Thus, the color deviation amount can be detected at high precision.

(Embodiment 7)

The apparatus constitution of the image forming unit of the color imageforming apparatus in this embodiment is same as in embodiment 1 shown inFIG. 1, and its description is omitted.

The function block diagram of the color image forming apparatus of thisembodiment is same as in embodiment 1 shown in FIG. 3, and itsdescription is omitted.

When the drawing fluctuation is a compound wave of a plurality ofperiods, the period and characteristic of the compound wave are mostinfluenced by the wave of the largest amplitude (drawing fluctuation)(not shown). In frequency analysis, each period is handled as a waveformsymmetric in a half period such as sine wave and cosine wave.Accordingly, regarding also the sine wave (cosine wave) having thelargest amplitude (drawing fluctuation) among the drawing fluctuations,in this sine wave (cosine wave), the registration pattern is drawn inthe same manner as in embodiment 2, and the detected values areaveraged. Thus, the color deviation amount can be detected at highprecision.

(Embodiment 8)

The apparatus constitution of the image forming unit of the color imageforming apparatus in this embodiment is same as in embodiment 1 shown inFIG. 1, and its description is omitted.

The function block diagram of the color image forming apparatus of thisembodiment is same as in embodiment 1 shown in FIG. 3, and itsdescription is omitted.

When the drawing fluctuation is a compound wave of a plurality ofperiods, the period and characteristic of the compound wave are mostinfluenced by the wave of the largest amplitude (drawing fluctuation)(not shown). In frequency analysis, each period is handled as a waveformsymmetric in a half period such as sine wave and cosine wave.Accordingly, regarding also the sine wave (cosine wave) having thelargest amplitude (drawing fluctuation) among the drawing fluctuations,in this sine wave (cosine wave), the registration pattern is drawn inthe same manner as in embodiment 3. Thus, the color deviation amount canbe detected at high precision.

(Embodiment 9)

The apparatus constitution of the image forming unit of the color imageforming apparatus in this embodiment is same as in embodiment 1 shown inFIG. 1, and its description is omitted.

The function block diagram of the color image forming apparatus of thisembodiment is same as in embodiment 1 shown in FIG. 3, and itsdescription is omitted.

In the foregoing embodiments 1 to 8, when drawing a pattern fordetection of period in the conveying drawing fluctuation perioddetector, first the pattern for detection of period is drawn, and theperiod is determined, then the registration pattern is drawn. In thisembodiment, the pattern for detection of period and registration patternare commonly used, or the registration pattern is drawn and detectedsimultaneously with the pattern for detection of period. If aregistration pattern drawn preliminarily is present at a positionconforming to the conditions of the foregoing embodiments 1 to 8, thecolor deviation amount is operated by using the result of measurement.It hence does not require the time for drawing the registration patternagain, and the color deviation amount can be detected at higherprecision.

Thus, according to the image forming apparatus of the invention, theregistration pattern is formed plural times at an arbitrary position inthe period determined by the conveying drawing fluctuation perioddetector, and the color deviation amounts obtained from the pluralregistration patterns are averaged to obtain the color deviation amount,and the precision of color deviation correction is enhanced, therebypresenting a color image forming apparatus capable of obtaining imagesof high print quality.

What is claimed is:
 1. A color image forming apparatus comprising: a) animage conveying unit for conveying an image by holding it on thesurface, b) a plurality of image forming units disposed for each colorfor forming the image corresponding to each color image information onsaid image conveying unit, c) a conveying drawing fluctuation perioddetector for detecting period of drawing fluctuation in said imageconveying unit, d) a registration pattern generator for controllingformation of registration pattern of each color on said image conveyingunit by said image forming units at the time of detection operation ofcolor deviation, and e) a color deviation detector for detecting colordeviation of the image from said registration pattern.
 2. The colorimage forming apparatus of claim 1, wherein said registration patterngenerator generates registration patterns plural times at an arbitraryposition in the period determined by said conveying drawing fluctuationperiod detector, and said color deviation detector detects the colordeviation amount by averaging the color deviation amounts obtained fromthe plural registration patterns.
 3. The color image forming apparatusof claim 2, wherein the registration patterns are formed plural times atequally divided positions in the period determined by the conveyingdrawing fluctuation period detector.
 4. The color image formingapparatus of claim 2, wherein the registration pattern is drawn at apoint of a small drawing fluctuation in the period determined by theconveying drawing fluctuation period detector.
 5. The color imageforming apparatus of claim 2, wherein the registration patterns areformed plural times at arbitrary positions in the period of the leastcommon multiples of the plural periods if there are plural periods ofdrawing fluctuation determined by the conveying drawing fluctuationperiod detector.
 6. The color image forming apparatus of claim 2,wherein the registration patterns are formed plural times at equallydivided positions in the period of the least common multiples of theplural periods if there are plural periods of drawing fluctuationdetermined by the conveying drawing fluctuation period detector.
 7. Thecolor image forming apparatus of claim 2, wherein the registrationpatterns are formed plural times at arbitrary positions in the period ofthe largest drawing fluctuation amount of the plural periods if thereare plural periods of drawing fluctuation determined by the conveyingdrawing fluctuation period detector.
 8. The color image formingapparatus of claim 2, wherein the registration patterns are formedplural times at equally divided positions in the period of the largestdrawing fluctuation amount of the plural periods if there are pluralperiods of drawing fluctuation determined by the conveying drawingfluctuation period detector.
 9. The color image forming apparatus ofclaim 2, wherein the registration pattern is drawn at the point of smalldrawing fluctuation in the period of the largest drawing fluctuationamount of the plural periods if there are plural periods of drawingfluctuation determined by the conveying drawing fluctuation perioddetector.
 10. The color image forming apparatus of claim 2, wherein theregistration pattern is drawn at the same time as the detection patternof said conveying drawing period detector.